High performance elastomeric polyurethanes prepared from isocyanate capped prepolymers by curing with methylenedianiline metal salt coordination complexes (MDA-CC) are known. For example, U.S. Pat. Nos. 3,755,261, 3,876,604 and 4,282,344 disclose the use of a MDA-CC, e.g., a 3:1 NaCl:MDA complex, as a blocked curative that can be mixed with prepolymers at low temperature to form curing compositions with long pot lives when kept at temperatures below the decomposition temperature of the MDA-CC. When heated above the activation temperature, i.e., the decomposition temperature of the coordination complex, the highly reactive MDA is released and reacts with the prepolymer to form high performance elastomeric polyurethane products.
Phase separation between soft and hard segments in a polyurethane elastomer is believed to play an important role in providing the excellent mechanical properties in high performance elastomers. The soft segment of a polyurethane is associated with the portion of the polymer derived from the polyol used in its preparation. Symmetrical prepolymers, such as symmetrical prepolymers prepared from a polyol and MDI capped, for example, MDI prepolymers having a low amount of fee MDI monomer (LFM prepolymers), are known to provide high performance elastomeric materials when cured with MDA-CC. The outstanding mechanical properties of these elastomers have been attributed in large part to the symmetrical MDI-MDA hard segment, which gives excellent phase separation.
Certain prepolymers used in the formation of high performance polyurethane elastomers are made from high molecular weight polyols and have a low isocyanate, e.g., isocyanate content of 4% or less. The high molecular weight polyol helps to enhance phase separation, but can also increase the possibility of “cold hardening”, a situation where at room temperature the material comprising the soft segment crystallizes and thus becomes harder. Plasticizers can be added to prevent cold hardening from occurring.
A variety of plasticizers are well known in polyurethane systems. For example, U.S. Pat. No. 5,872,193 discloses a polyisocyanate prepolymer composition prepared by reacting diphenylmethane diisocyanate with an active hydrogen containing compound having a number average molecular weight of at least about 2000, wherein the reaction is carried out in the presence of a diluent capable of maintaining the average viscosity of the resulting prepolymer below about 500 cps at 25° C. The diluent is typically a plasticizer and a large number of useful plasticizers are listed, including adipates, phthalates, benzoates, azelaic acid derivatives, citrates, epoxy derivatives of naturally oils such as epoxidized soybean oil and epoxidized linseed oil, glycol esters, polyethylene glycols, oleic acid derivatives and various phosphoric acid esters and others. Specifically named phthalic acid derivatives include dimethyl phthalate, dibutyl phthalate, butyl octyl phthalate, diisohexyl phthalate, heptyl nonyl phthalate, diisooctyl phthalate, bis(2-ethylhexyl) phthalate, (n-hexyl, n-octyl, n-decyl) phthalate, n-octyl, n-decyl) phthalate, diisodecyl phthalate, diundecyl phthalate, ditridecyl phthalate, butyl cyclohexyl phthalate, butyl benzyl phthalate, alkyl benzyl phthalate, 7-(2,6,6,8,-tetramethyl-4-oxa-3-oxo-nonyl) benzylphthalate, bis(2-butoxyethyl) phthalate, dicyclohexyl phthalate, and diphenyl phthalate, among others. Among the useful polyesters included are adipic acid polyesters, azelaic acid polyesters and sebacic acid polyester. Very highly preferred diluents are dialkyl phthalates. However, U.S. Pat. No. 5,872,193 provides no examples in which a specific plasticizer is named.
Phthalates and adipates are used in commercially available MDA-CC curing agents. U.S. Pat. No. 3,888,831 discloses that the curing of amine-curable polymers, such as polyurethane prepolymers, can be accelerated in the presence of plasticizers such as tetra-ethylene glycol bis-2-ethyl hexanoate and dipropylene glycol dibenzoate.
U.S. Pat. No. 4,207,128 discloses a process for producing microporous coatings comprising (1) mixing an NCO-prepolymer with 3 to 40% by weight based on the NCO-prepolymer of a plasticizer; (2) adding about 3 to 300 weight % of water to form a water-in-oil emulsion; (3) adding a polyamine in a quantity equivalent to the isocyanate groups; (4) applying a coating of the water-in-oil emulsion to a substrate; and (5) hardening the coating by evaporating the chemically unbound water from said coating. Plasticizers include phenol esters of long-chain sulphonic acids and phthalic acid esters, phosphoric acid esters, monocarboxylic acid esters, and chlorinated aromatic hydrocarbons.
Dipropylene glycol dibenzonate, commercially available, for example, as Benzoflex™ 9-88, is widely used as a plasticizer in preparing softer cast polyurethane elastomers. Although dipropylene glycol dibenzonate, and to a lesser extent other plasticizers, have found commercial success in elastomeric polyurethane production, there is room for improvement in plasticized compositions used in preparing high performance polyurethane elastomers. For example, there is a need for curing compositions, especially for forming high performance elastomers from LFM prepolymers and MDA-CC, with improved NCO stability that result in elastomers with less color and better overall physical properties.
Surprisingly, it has been found that particular plasticizers, when added to isocyanate capped prepolymers, provide compositions with increased prepolymer stability, improved processing characteristics, especially when curing with MDA-CCs to form polyurethane elastomers, and result in polyurethane elastomers with enhanced physical properties, relative to compositions comprising other similar plasticizers.